The surface energy/tension of a substance can be explained different types of interaction. Dispersive and polar interactions are differentiated. The surface tension/energy of a substance is made u pof dispersive and polar parts. Polar interactions are strong and long-range interactions between molecules based on electrostatic attraction. In many molecules, the charge is not evenly distributed, but is present in the form of partial charges. Unequal charges attract each other, and this leads to strong interactions between the charges.
In polar interactions, a distinction is made between different contributions, which are of different strength and have different ranges. The strongest interactions are the interactions between ions. With a distance dependence of 1/r these are also very long-range. Then follow the interactions between a permanent dipole and ions with a distance dependence of 1/r². Significantly weaker are the dipole-dipole interactions. Here the distance dependence depends on whether the dipoles can rotate freely or not. A special case of dipole-dipole interactions are the hydrogen bonds. These are very targeted interactions that act between hydrogen atoms and negative partial charges. These strong interactions are the reason for the high surface tension of water. If there are no permanent charges, they can be induced. These dispersive forces are much weaker than the polar ones, but they play an important role e.g. in plastics.
Strong interaction forces are especially important when it comes to stable compounds. While disperse interactions are also possible without permanent dipoles, charges are needed for polar interactions. Especially so-called low-energy surfaces with a low surface energy and a low polar fraction often cannot be further processed without pre-treatment. One possibility of pre-treatment is the plasma treatment, in which reactive species remove the finest impurities from the surface and additionally functionalise the surface by the addition of polar groups. The polar groups contribute to the polar interactions with their dipole moment, which is also reflected in the polar fraction of the surface energy. In many applications, it has been shown that only a sufficiently large polar fraction enables surfaces to be wetted with many coatings, adhesives and other substances.
There are no substances with only polar interactions because dispersive interactions occur between all atoms and molecules. But there are substances which don’t have any polar groups. Therefore theri surface tension/energy is purely dispersive.
The comparison oft he ratio between dispersive and polar parts oft he surface energy/tension fort wo phases allows a prediction oft he adhesion. The closer the ratios match the more interactions are possible between the phases. Therefore the adhesion ist o be expected higher (see figure 1 and 2). A small interfacial energy/tension shows a high potential for interaction between two phases.« Back to Glossary Index